1. Field of the Invention
The present invention relates to a mask for X-ray exposure and a fabrication process of this mask and, more particularly, to a mask structure for X-ray exposure capable of high-accuracy exposure transcription in X-ray lithography for transferring an electronic circuit pattern on a mask surface onto a wafer surface with use of X-rays, for example, in the wavelength range of approximately 2 to 150 .ANG. and thereby fabricating semiconductor devices such as ICs or LSIs, micromachines, microoptics, and so on.
2. Related Background Art
In recent years pattern line widths of integrated circuits tend to decrease with increase in density and speed of semiconductor integrated circuit and it is said that they were reduced approximately 70% for three years These large-capacity memory elements are transcribed from a mask to a semiconductor substrate, using light in the region of from the near-ultraviolet light to the far-ultraviolet light, but the processing line widths of semiconductor device that can be processed with the light in these wavelength regions are approaching the limitation. Also, a reduction in focal depth inevitably occurs with a decrease in the exposure wavelength or with an increase in the numerical aperture of projection optical system Thus, exposure apparatus utilizing X-rays capable of higher-resolution printing are proposed with an increase in integration of semiconductor device. This X-ray lithography technology is highly expected as one capable of solving the issues of resolution and focal depth described above at the same time.
An X-ray mask used for above X-ray exposure normally has such structure that fine patterns of X-ray absorber and alignment patterns used for alignment are formed on an X-ray transmitting film made on an appropriate support frame such as a silicon wafer and that the support frame is further fixed on a mask frame of boro-silicate glass (e.g., Pyrex) or the like.
In carrying out patterning on an X-ray photoresist on the substrate with such an X-ray mask structure, appropriate exposure doses for attaining a resist pattern in the same size as that of a pattern on the mask differ depending upon the size of pattern. This means that the so-called mask size linearity is poor. In general, it is reported that low-contrast masks are advantageous to obtaining finer patterns. On the other hand, the transmittance of X-ray increases with decreasing mask contrast, which will cause such problems that a film reduction occurs in the resist pattern after development, that an undesired residual film appears after development, and so on, especially in the case of patterns of relatively large size. Because of this, It is described, for example in Japanese Patent Publication No. 7-70466, that the film thicknesses of X-ray absorber pattern are changed depending upon the pattern sizes. It was, however, not easy to process X-ray absorber patterns of the same material in different thicknesses and at high accuracy on the same plane in view of control accuracy of etch rate.